Cellulose and lignin from plants are among the most prominent renewable carbon sources. These molecules are comprised in plants as lignocellulose structures; fibers of cellulose polymers entangled in a network of lignin polymers. Lignocellulose is composed mainly of cellulose, hemicellulose and lignin. Lignin may make up to 25% of the lignocellulosic biomass. For fermentable sugar production, Miscanthus grass species, wood chips and the byproducts of lawn and tree maintenance are some of the more popular lignocellulosic materials. Corn stover, Panicum virgatum (switchgrass) and Miscanthus are the major biomass materials being studied today, due to their high productivity per acre. Cellulose, however, is contained in nearly every natural, free-growing plant, tree, and bush, in meadows, forests, and fields all over the world without agricultural effort or cost needed to make it grow.
The cellulose fraction of various lignocelluloses is a uniform structure consisting of β-1,4 linked glucose units. However, the biodegradability of cellulose may vary between plants, depending on the strength of association of the cellulose with other plant compounds. The composition and proportion of hemicellulose and lignin are highly dependent on the nature of the material. There is more lignin in softwoods (for example, spruce) than in hardwoods (for example, willow) or agricultural residues (for example, wheat straw or sugarcane bagasse), which makes softwood a particularly challenging material for ethanol production. The major hemicellulose component of hardwood and agricultural residues is xylan, while that of softwood is mostly mannan.
There are essentially two ways of producing ethanol from cellulose. First there are cellulolysis processes which consist of hydrolysis of sometimes pretreated lignocellulosic materials, using enzymes to break complex cellulose into simple sugars such as glucose, followed by fermentation and distillation. Second, there is also gasification that transforms the lignocellulosic raw material into gaseous carbon monoxide and hydrogen. These gases can then be converted to ethanol by fermentation or chemical catalysis.
The process involving cellulolysis can typically be divided into several stages: first, there may be a “pretreatment” phase, to make the lignocellulosic material such as wood or straw more amenable to hydrolysis. A hydrolysis (the actual cellulolysis) step, to break down the molecules into sugars followed by the separation of the sugar solution from the residual materials, notably lignin, followed by microbial fermentation of the sugar solution and distillation to produce roughly 95% pure alcohol.
Although lignocellulose is the most abundant plant material resource, its susceptibility has been curtailed by its rigid structure. As the result, an effective pretreatment is needed to liberate the cellulose from the lignin seal and its crystalline structure so as to render it accessible for a subsequent hydrolysis step. By far, most pretreatments are done through physical or chemical means.
Physical pretreatment is often called size reduction to reduce biomass physical size. Chemical pretreatment is to remove chemical barriers so the enzymes can have access to cellulose for enzymatic destruction.
To date, the available pretreatment techniques include acid hydrolysis, steam explosion, ammonia fiber expansion, organosolve, sulfite pretreatment to overcome recalcitrance of lignocellulose, alkaline wet oxidation and ozone pretreatment.
In acid-catalyzed pretreatment, the major part of the hemicellulose is degraded, and the cellulose has to be hydrolyzed by the use of cellulases, whereas in alkali-catalyzed pretreatment, part of the lignin is removed, and in addition to cellulases, hemicellulases are also needed to hydrolyze the remaining polysaccharides.
The complete hydrolysis of cellulose and hemicellulose requires a well-designed cocktail of enzymes consisting of endoglucanases, cellobiohydrolases, β-glucosidases, xylanases, mannanases and various enzymes acting on side chains of xylans and mannans.
Due to the recalcitrant structure of lignocelluloses, a pretreatment step may be required prior to enzymatic hydrolysis in order to make the cellulose more accessible to the enzymes. Despite of the above developments, most pretreatment processes are not effective when applied to feedstocks with high lignin content, such as forest biomass. The present invention addresses this problem.